Hitherto, as a proofing method for color prints, a method of performing proof printing using a print proofreading machine or a main printing machine has been used. However, proof printing by a proofing method is expensive, printing requires a long period of time, and also there is a limit on the reliability thereof.
On the other hand, various color proofing methods by photographic process having been employed as a simple and easy method in place of the aforesaid printing methods.
As the color proofing method by photographic process, there is a color proofing method using a photopolymer (i.e., pre-press proof), an overlay method and a surprint method.
In the overlay method, plural color proofing sheets each having separation images of each color on a transparent support are prepared and color proofing is performed by superposing these sheets (the assembly of the sheets thus obtained is called a "color test sheet").
The overlay method has the advantages that the method is very simple and inexpensive and the method can be used for continuous inspection by superposing 2 single color sheets or 3 single color sheets only each time, but has the disadvantages that the color test sheet obtained becomes dark to some extent by the superposed synthetic resin sheets and also the incident light is reflected by several sheets to give luster, whereby the impression received from the color test sheet is very different from the quality impression of a print printed by an ordinary printing machine.
On the other hand, in the surprint method, plural single-color layers are superposed on a single support and for that purpose, various kinds of toners are applied onto a common opaque film base or plural light-sensitive layers each corresponding to each color are formed, in succession, on an opaque film base. Details of these methods are described, for example, in U.S. Pat. Nos. 3,671,236 and 3,136,637.
The surprint method has the advantage that the color density is not influenced by the synthetic resin base. Also, the surprint method is more similar to the original printing method and further has the advantage that the occurrence of color stain in the case of superposing sheets as in the aforesaid overlay method is inhibited.
Furthermore, a method of imagewise exposing each of the light-sensitive transfer materials each having, in succession, a peelable layer composed of an organic polymer, a coloring material layer, and a light-sensitive layer on a temporary support, developing the light-sensitive transfer material to form color images on the peelable layer, and then transferring the color images onto an optional support (permanent support) each time by using an adhesive is well known as described in Japanese Patent Publication Nos. 15326/71 and 441/74. This method has the advantage of being usable for various operations such as an overlay type and surprint type as color proof, but has the disadvantage that the method is troublesome, since an adhesive must be used at each transfer of color images and also since it is difficult to keep the preciseness of matching the positions of color images when transferring images of each color.
For removing the aforesaid troublesome defects of these methods, a method of transferring images formed by applying heat and pressure onto a permanent support is disclosed in Japanese Pat. Application (OPI) Nos. 41830/72, 9337/73, and 5101/76 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"). In particular, Japanese Pat. Application (OPI) No. 5101/76 discloses that a heat-fusible polymer layer is formed on a permanent support as adhesive and also Japanese Pat. Application (OPI) No. 41830/72 discloses a method of directly transferring images onto a permanent support such as an art paper or a coated paper.
However, these methods have various disadvantages. That is, these are the problems that the final image transferred onto a permanent support is right and left reversed and also when a heat-fusible polymer is used as an adhesive, an increase in transfer temperature is required, since the melting point thereof is generally high, whereby the dimensional stability of support is reduced by the influence of heat to cause discrepancies in the matching of the transferred position of each color images. On the other hand, when a heat-fusible polymer having low melting point is used as an adhesive, the surface of the images transferred becomes adhesive and thus the surface is liable to be injured.
For overcoming the aforesaid difficulty, a method of once transferring images onto a temporary image-receiving sheet before transferring the images onto a permanent support was previously proposed in Japanese Pat. Application (OPI) No. 97140/84 (corresponding to U.S. Pat. No. 4,482,625) by the same assignee. That is, according to this method, a temporary image-receiving sheet composed of a support having formed thereon an image-receiving layer composed of a photopolymerizable material is prepared, images of each color are once transferred onto the temporary image-receiving sheet before being transferred onto a permanent support, the color images thus formed on the image-receiving sheet are then retransferred onto a permanent support, and the images thus transferred are overall light-exposed to harden the image-receiving layer composed of the photopolymerizable material thus transferred onto the permanent support.
The above-described image transfer method using a temporary image-receiving sheet (hereinafter referred to simply as an image-receiving sheet) is very effective for solving the aforesaid problem. That is, by utilizing the aforesaid method, a correct image for an original can be obtained on a permanent support. Since the photopolymerizable image-receiving layer of the image-receiving sheet contains an ethylenic polyfunctional monomer, the photopolymerizable image-receiving layer itself is soft, image transfer is possible at a low temperature, and after transferring images thereon, the image-receiving layer having transferred images thereon can be easily hardened by overall light exposure. That is, the aforesaid methods have such advantages that adhesion does not occur after transferring images and the resistance to injury of the final images obtained is high.
In the image-receiving sheet used for the method described in Japanese Pat. Application (OPI) No. 97140/84 described above, the adhesive strength between the photopolymerizable image-receiving layer (i.e., photopolymerizable adhesive layer) and the support is very high in the unexposed state. Accordingly, in transferring the photopolymerizable adhesive layer having images received thereon onto a final support, peeling stripes are liable to form on the surface of the transferred image-carrying photopolymerizable adhesive layer if the support of the image-receiving sheet material is peeled off before applying light exposure. For overcoming this problem, Japanese Pat. Application (OPI) No. 97140/84 provides a method wherein after intimately contacting an image-receiving photopolymerizable adhesive layer onto a final support, the photopolymerizable adhesive layer is first hardened by applying thereto overall light exposure whereby the adhesion between the photopolymerizable adhesive layer and the support for the image-receiving sheet is reduced, and then the image-receiving sheet support is removed.
However, these surprint methods have disadvantages in that they are expensive and they cannot satisfy the need for both high quality and low cost in color proofing by these photographic processes.